Capintec CAPTUS 3000, Owner'S Manual, Page: 70 / 338

CAPINTEC, INC. CAPTUS
®
3000
About Auto Calibration
Calibration is done using Cs137 and Eu152 isotopes. First, calibration is performed with
Cs137 where the procedure makes use of its 662 keV and the 32 keV peaks to automatically
adjust the high voltage, gain, and zero offset (as required by MCA card) to provide a
calibration of approximately 2.0 keV per channel in the MCA. Next, Eu152 provides multiple
gammas over an energy range of 41 keV to 1,408 keV. The CAPTUS
®
3000 software
provides a point-by-point energy calibration using these gammas. This function corrects for
any non-linearity in response of the NaI detector and provides accurate identification of the
energy of the peaks. It is important to complete the calibration of your system using Eu152.
The ROI’s in the Thyroid Uptake and Lab Tests are based on the energy ranges defined by
the Eu152 calibration. In addition, the automatic peak identification program, which is
accessed in the Wipe Test and MCA, requires the Eu152 energy calibration in order to work
properly.
Unless an error message is displayed, the software will have succeeded in finding a set of
adjustments, which places the 662 keV gamma peak to within ±2 channels of the theoretical
value for a calibration of 2.00 keV per channel. The software has preset limits that prohibit
operation outside of appropriate voltage, gain, and zero adjust values.
The Auto Calibration Test also calculates detector resolution by determining the FWHM (full
width-half maximum) area of the peak, and dividing by the peak centroid location. For peaks
such as the 662 keV peak of Cs137 whose shape is symmetrical or appears to follow a
Gaussian distribution, the FWHM is generally close to 2.35
σ
. For flat-faced detectors the
resolution should be <9.5 percent measured at the Cs137 662 keV peak. For well detectors
the resolution should be <10.0 percent. Detectors do age with use, and this will result in a
very slight increase (1% to 3%) in resolution over a period of several years. A rapid increase
in percent resolution may indicate a problem with the detector assembly.
Note: Monitoring the stability of the resolution over time is one of the best indications of the
overall system performance.
CONSTANCY TEST
CAUTION: Calibration is recommended before making any measurements. Refer to
the
AUTO CALIBRATION section on page 5-5.
This test compares the Cs137 standard source activity entered in the setup with the activity
that it measures. The activity entered in the setup is decay corrected by the software. The
counts measured in the Constancy test are converted to activity using efficiency values from
the Isotope Library. The deviation between the calculated and measured activity is the
percent error and should be within ±10%. If the deviation is outside ±10%, it will be indicated
as HIGH. NaI detectors are very sensitive and any radioactive sources even in neighboring
rooms may affect the results.
•
Perform SETTING UP QA PARAMETERS on page 5-2 before performing the
Constancy Test the first time.
•
Perform Auto Calibration before Constancy Test.
5-10 QUALITY ASSURANCE July 14